Photo by Pixabay
(Click here to listen to an audio version of this blog … Photoperiodism and the first hints of winter
This coming weekend we leave Daylight Savings Time (DST) behind and return to Standard Time (ST). I have talked about the ideas behind DST before and also discussed some of the problems that occur especially in the Spring transition (the great “spring ahead”) from ST to DST (see Signs of Spring 2, March 14, 2019). Does going to and from DST make sense? Does it convey any particularly important benefits? Probably not, but amazingly most people are very used to it and would probably miss it!
The change in our clocks from DST to ST does nothing to alter the total amount of daylight we get in a day. It does, however, re-shuffle our perceptions of lengths of day and night. Mornings in ST are just a little more sunlit than they were before the clock shift, and afternoons and evenings are a little bit less.
In reality, our real concern with time and times of day, should be the realization that we are racing toward the Winter Solstice (December 21 this year). The Winter Solstice represents the day when the North Pole is tipped as far away from the sun as it can go (and that angle and even the direction of that tipping has not been a constant through geological time!). On the Winter Solstice we will have just over an hour less sunlight than we do today (only 9 hours and 17 minutes and 4 seconds compared to today’s (November 4, 2021) 10 hours, 19 minutes and 13 seconds). Do you remember the Summer Solstice back on June 20? That was the day when the North Pole was tilted as directly toward the sun as it can go! We had 15 hours and 3 minutes and 42 seconds of sunlight! How did we ever get to sleep that day?
Anyway, our shifts from DST to ST do not affect organisms other than humans, but the on-going, seasonal transition of day length and darkness do affect almost every plant and animal species around us.
Photo by D. Sillman
I used to watch my box turtle (the late, great Spider the Turtle!) get sleepier and sleepier as the days got shorter. I would start to keep him exclusively in the house as Fall temperatures began to drop, but the decreasing time of sunlight inevitably pulled him into a torpor that would have led all the way into hibernation if he had been living outdoors. Even by moving him into the constant temperature environment of my dining room (where his fall/winter terrarium was located), he still could not resist the hardwired physiological changes triggered by the passing season. He would eat his last nightcrawler of the year in mid to late October and then drop into a slow, half-sleep that last almost five months. He wouldn’t have another meal (usually the really expensive, first available strawberries!) until the first or second week in March.
Taz. Photo by D. Sillman
My housecats (the late, great and terribly missed Taz and Mazie) were incredible hair producers! Brushing them or even just petting them in the spring or summer generated great handfuls of shed hair. The shortening day lengths, though, did slow down the rates of their amazing synthesis of hair a little bit. In an outside cat, the production of hair without the shedding loss would result in a thick, insulating winter coat. Indoor cats, though, don’t usually build up quite as heavy a hair layer even if they have an owner who seldom
Mazie. Photo by D. Sillman
turns up the thermostat.
Shortening day lengths are the prime stimuli for many of our summer birds to begin their southward migrations. They need to fatten up and leave their summer ranges well before food supplies are gone and cold weather has settled in. Getting a physiological nudge from the shortening days to start their pre-migration prep before getting hammered by plunging temperatures or blowing snow is a great evolutionary advantage!
Around here the hummingbirds, Swainson’s hawks, orioles, tanagers, house wrens, chipping sparrows, grosbeaks, flycatchers, swallows and more that have graced us with their summer presence will join the clouds of birds passing through from even more northern regions and fly out of Northern Colorado in flocks of many thousands. Each species is tuned to their own specific day length stimuli, each one has been working hard to put on their fat layers, and each are headed out toward their own, specific overwintering regions.
Photo by Mwanner. Wikimedia Commons
There are also a small number of far-north-dwelling birds that will follow their own daylength cues and will soon begin to arrive here in Colorado to join us in our winter. The sandhill cranes have been flying in daily down in the San Luis Valley and the Great Sand Dunes National Park in the southcentral and southeastern parts of the state. Canada geese and snow geese will begin arriving in the next few weeks to fill ponds and lakes all over the state. Over at the nearby Arapaho Natural Area, American tree sparrows and cackling geese join Canada geese, snow geese and Oregon juncos in their overwintering habitats. Last year, a small group of Oregon juncos showed up in our backyard during several of the warmer, winter days. They reminded me of the flocks of dark-eyed juncos that overwintered at our feeders back in Pennsylvania. The October arrival of those Pennsylvania “snow birds” and their departure in March were vivid signs of winter and of spring.
One hundred species of birds have been identified in Arapaho Bend Natural Area. Forty-five of these species are year-round residents while forty others are only seen in the summer. Fifteen species are migratory including ten that pass through the area quickly in either the fall or spring and five others (the ones listed above) that stay for the winter.
American robin. Photo by D. Sillman
Up in Pennsylvania, American robins were the archetypical birds of fall and spring migration (even though many of them have settled into our larger cities as year round residents! (see Signs of Spring 4, January 30, 2013)). Robins do a very different type of winter migration here. They have spent most of their summer in their breeding ranges up the mountains but come down to the relative shelter of the “tree island” towns on the plains (like Greeley) when the cold weather begins to set in. Last year, we had dozens of robins in our backyard and especially at our birdbath on sunny winter days.
Even humans respond to the shortening day lengths, although the pattern of this response is not always the same in all people. Thyroid hormone seems to either go up or get more active in most people in the winter. Logically, it stimulates metabolic rate and heat production especially through the metabolism of carbohydrates and possibly also stimulates metabolic heat generation in the body’s brown fat deposits. Cortisol levels also go up in the winter probably acting to shift metabolism over to using fats (both stored and dietary) for energy. But all of these responses are really quite muted compared to other mammalian species that lack all of those human technologies that protect us from the great stresses of winter.
Photo by D. Sillman
Plants have all sorts of photoreceptor proteins that respond to day length. These proteins are especially sensitive to the duration of the dark periods. It is the length of the night that drives plant species to flower, or make seed, or senesce. The cessation of chlorophyll production in deciduous tree leaves and its accelerated breakdown reveals the formerly hidden accessory pigments and generates the beautiful color displays of the autumn trees (see Signs of Fall 3, October 8, 2020). W. D. Hamilton (the famous “Bill Hamilton” who was a great friend and colleague of Richard Dawkins) suggested that these accessory leaf pigments are not just revealed as the chlorophylls fade but that the lengthening nights actually accelerate their production. These pigments then act as deterrents to insects and prevent their overwintering and egg laying in the tissues around the senescing leaves.
So, our clocks are changing, but more importantly, the light parts of the days are getting shorter. The animals around us are shifting about, and the plants are beginning to enter their winter resting phases. Winter is getting close!